Resilient provider link state bridging (plsb) virtual private lan service (vpls) interworking

ABSTRACT

A method of peer interfacing a Link-State controlled network domain with an Ethernet Bridging controlled network domain. A pair of peer attachment points are provided between the Link-State controlled network domain and the Ethernet Bridging domain. The peer attachment points are respective endpoints of a set of one or more LAN segments defined within the Ethernet Bridging domain. The set of LAN segments are represented as a virtual node in the Link-State controlled network domain. The virtual node is represented in the Link-State controlled network domain as connected to each of the peer attachment points via a respective virtual link. The virtual links are configured such that frames to or from an address in the Link-State controlled network domain are forwarded over a tree passing through only one of the peer attachments points.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/334,013, allowed, which is based on, and claims priority of BritishPatent Application No. 0802371.5 filed Feb. 9, 2008.

MICROFICHE APPENDIX

Not Applicable.

TECHNICAL FIELD

The present invention relates to management of traffic forwarding inframe networks, and in particular to methods of interfacing ProviderLink State Bridging (PLSB) and Virtual Private LAN Service (VPLS)network domains.

BACKGROUND OF THE INVENTION

Network operators and carriers are deploying frame-switchedcommunications networks in place of circuit-switched networks. Inframe-switched networks such as Internet Protocol (IP) networks, IPframes are forwarded according to routing state stored at each IP routerin the network. Similarly, in Ethernet networks, Ethernet frames areforwarded according to forwarding state stored at each Ethernet switchin the network. The present invention applies to communications networksemploying any Protocol Data Unit (PDU) based network and in thisdocument, the terms “frame” and “frame-switched network”, “routing”,“frame” and “frame-based network”, “forwarding” and cognate terms areintended to cover any PDUs, communications networks using PDUs and theselective transmission of PDUs from network node to network node.

Multicast forwarding of data frames (where frames are sent from a sourcenode to multiple destination nodes more or less simultaneously) is ofincreasing importance as demand for services such as Internet ProtocolTelevision (IPTV) and Video on Demand (VoD) grows.

Protocols such as Intermediate System-Intermediate System (IS-IS) orOpen Shortest Path First (OSPF) are used to disseminate network topologyinformation used to calculate paths for forwarding frames from aplurality of source nodes to one or more destination nodes, typicallythrough one or more intermediate nodes, and to install the forwardingstate required to implement those paths. OSPF and IS-IS are run in adistributed manner across nodes of the network so that, for example,when a topology change occurs in the network such as a node or linkfailure, this information is flooded to all nodes by the protocol'soperation, and each node will locally recompute paths to circumvent thefailure based on a consistent view of network topology.

In Ethernet networks, Provider Backbone Transport (PBT), also known asProvider Back-Bone Bridging-Traffic Engineering (PBB-TE), as describedin Applicant's British patent number GB 2422508 is used to provide aunicast Ethernet transport technology. Provider Link State Bridging(PLSB) as described in Applicant's co-pending U.S. patent applicationSer. No. 11/537,775 will be used to provide a multicast transportcapability for Ethernet networks using IS-IS to set up unicast paths andmulticast trees in the network. Both above patent documents are herebyincorporated by reference.

Many network operators have deployed Multi Protocol Label Switching(MPLS) as their frame switched network transport technology, with anoverlay technology called Virtual Private LAN Service (VPLS) providingthe infrastructure for customer any-to-any connectivity (E-LAN services)delivered over restricted (typically metro scale) network domains. VPLSis an Ethernet LAN emulation provided over MPLS. Within this document,the terms VPLS and Ethernet LAN segment are used interchangeably todescribe the service offered to the end-customer.

A problem with VPLS is that it scales poorly, in particular becausecustomer Media Access Control (C-MAC) addresses are exposed to the VPLSdomain. Further, VPLS constructs a full mesh of pseudo-wires betweenevery node with a point of presence for any specific service, so thetelemetry associated with a VPLS service instance scales in proportionto the square of the number of end points. Finally the full mesh ofpseudo wires means that any flooding of frames is inefficient, as allframe replication must be performed at the ingress to the pseudo-wiremesh. In cases where the number of pseudo-wires exceeds the number ofphysical links traversed at a given point, multiple copies of the sameframe will be sent on each physical link.

One approach to mitigating this scaling problem is to use HierarchicalVPLS (H-VPLS), which uses multiple hub and spoke architectures at theedges to contain the size of the fully-meshed transport core, and thuslimit the number of transport connections required. This approach hasthe penalty alluded to above, in that the gateways between edge and coreare exposed to the full range of C-MAC addresses, which was already asevere scaling limitation. It also introduces additional complexity toaddress resiliency issues as it requires multi-homing of the spokes ontothe core mesh.

An increasingly preferred approach to the scaling problems of VPLS is touse Provider Backbone Bridges (PBBs)—standardized as IEEE 802.1ah—at theedges of the VPLS core, to separate the C-MAC address spaces from theoperator backbone MAC (B-MAC) address space through encapsulation. Inthis way, a VPLS domain is typically exposed to a small number of B-MACaddresses summarizing a much larger set of C-MAC addresses which wouldtypically be found on a customer LAN segment.

However the deployment of PBB overlaid on existing VPLS sufferslimitations in that interworking between a PBB Network and legacy ports(i.e. where the peer VPLS Provider Edge router, PE, is not configured tosupport Backbone Edge Bridging) presents numerous challenges andcomplexity, and in any case the combination of VPLS and PBB only has thecapability to address some of the scaling issues of VPLS.

An alternative approach is to migrate towards a PLSB core network asPLSB overcomes many of the shortcomings of VPLS with respect tomulticast efficiency, resiliency and ease of provisioning. It isdesirable to be able to do this without perturbing existing deployedcustomer facing VPLS ports and at the same time maximizing theutilization of deployed assets. Similarly where VPLS has been deployedin the core and the decision has been made to deploy PLSB in the metro,it is desirable to use the deployed MPLS/VPLS capacity until such pointas network load and economics mandates direct interconnect of subtendingPLSB metro area networks.

Therefore a means of resilient and efficient interconnect of PLSB andexisting VPLS is highly desirable. This needs to be true where VPLSsubtends the Link-State controlled domain (User Network Interface (UNI)interconnect) and where VPLS simply lends transit capacity to Link-Statecontrolled domain (Network Network Interface (NNI) interconnect).

SUMMARY OF THE INVENTION

Thus, an aspect of the present invention provides a method of peerinterfacing a Link-State controlled network domain with an EthernetBridging controlled network domain. A pair of peer attachment points areprovided between the Link-State controlled network domain and theEthernet Bridging domain. The peer attachment points are respectiveendpoints of a set of one or more LAN segments defined within theEthernet Bridging domain. The set of LAN segments are represented as avirtual node in the Link-State controlled network domain. The virtualnode is represented in the Link-State controlled network domain asconnected to each of the peer attachment points via a respective virtuallink. The virtual links are configured such that frames to or from anaddress in the Link-State controlled network domain are forwarded over atree passing through only one of the peer attachments points.

In some embodiments the Ethernet Bridging domain subtends the Link-Statecontrolled domain and exchanges frames at the C-MAC layer (UNIinterworking), and in other embodiments the Ethernet Bridging domainpeers with the Link-State controlled domain at the B-MAC layer (NNIinterworking).

For the UNI interworking scenario, at least two attachment points areprovided between the PLSB domain and each subtending VPLS domain. Eachattachment point comprises a VPLS gateway interconnected with a PLSBgateway, the VPLS gateway being an end-point of one or more sets ofvirtual LAN segments defined within the VPLS domain, each virtual LANsegment corresponding to a customer service instance. Each set ofvirtual LAN segments (VLANs) is represented as a virtual node of thePLSB domain. The modelling of virtual node connectivity to therespective PLSB gateway of each attachment point uses a respectivevirtual link with specific metric assignment, such that every path onthe shortest path tree computed by the PLSB domain between the set ofvirtual LAN segments and every destination address in the PLSB domainwill traverse the respective PLSB gateway of only one of the at leasttwo attachment points at any given time.

For the NNI interworking scenario, VPLS can be configured a priori toprovide at least one VPLS LAN segment, each of which has one or moreunique physical points of interconnect with each PLSB domain. Each ofthe VPLS LAN segments supports multiple infrastructure “virtual LANsegments”. IS-IS discovery procedures will correctly model each VPLS LANsegment as a topology component in the PLSB network and will install MACfiltering at the PLSB/VPLS boundary nodes accordingly. Failure of anycomponent of a VPLS LAN segment will be reflected in the PLSB IS-ISrouting system, and connectivity will be rerouted to use the survivingconnectivity accordingly.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the present invention will becomeapparent from the following detailed description, taken in combinationwith the appended drawings, in which:

FIGS. 1A and 1B are block diagrams schematically illustrating a methodof interfacing VPLS and PLSB domains in accordance with a firstrepresentative embodiment of the present invention;

FIG. 2 is a block diagram schematically illustrating a method ofinterfacing VPLS and PLSB domains in accordance with a secondrepresentative embodiment of the present invention;

FIG. 3 is a block diagram schematically illustrating a potential loop;and

FIG. 4 is a block diagram schematically illustrating a method ofinterfacing VPLS and PLSB domains in accordance with a fifthrepresentative embodiment of the present invention, which avoids thepotential loop of FIG. 3.

It will be noted that throughout the appended drawings, like featuresare identified by like reference numerals.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention provides a method for management of trafficforwarding in frame networks, and in particular to methods ofinterfacing Link State protocol controlled network domains and VirtualPrivate LAN Service (VPLS) network domains. Embodiments of the inventionare described below, by way of example only, with reference to FIGS.1-4.

PLSB as described in Applicant's co-pending U.S. patent application Ser.No. 11/537,775 provides a link state control plane for Ethernet networksusing IS-IS to disseminate information that permits local computationand set up unicast paths and multicast trees in the network. The abovepatent document is hereby incorporated by reference. Like PBB, PLSB usesencapsulation to hide the C-MAC address spaces from the backboneoperator network, facilitating scalability and security. PLSB is a“routed” infrastructure technology; and the IS-IS control plane makesnodes aware of the network topology, and the route to any specific B-MACaddress. Consequently Spanning Tree Protocol (STP) and auto learning offorwarding state are not used on PLSB nodes. Furthermore, instead of theconventional broadcasting of frames with unknown destination addresses,PLSB nodes discard any frames with unknown addresses. A useful andimportant property of PLSB when interfacing with existing Ethernet orEthernet “emulation” is that the forward and reverse paths between anytwo points follow the same route or are “congruent”. This at a microlevel corresponds to existing Ethernet practice.

As shown in FIGS. 1A-B, PLSB and VPLS networks 2, 4 can beinterconnected using suitable PLSB gateways 6 (eg Backbone Edge Bridges,BEBs, or Backbone Core Bridges, BCBs) and conventional VPLS ProviderEdge routers (PEs) 8. According to the present invention, scalabilityand resiliency issues inherent to VPLS are overcome by using VPLS merelyto establish virtual LAN (VLAN) segments 10 between VPLS PEs 8. Networkintelligence, in terms of traffic forwarding, failure recovery, and loopprevention is provided exclusively within the PLSB domain.

FIG. 1A illustrates an embodiment implementing UNI interworking, inwhich a client system (CS) 12 in the VPLS domain is interconnected witha destination address (DA=X) 14 in the PLSB domain. FIG. 1B illustratesan embodiment implementing NNI interworking, in which a path is set upbetween a source address (SA) 16 and a destination address (DA=X) 14 ina PLSB domain 2 which traverses the VPLS domain 4. As may beappreciated, in the embodiment of FIG. 1B, the source and destinationaddresses 14, 16 may be in the same PLSB domain, or in different PLSBdomains, as desired.

Normally, both UNI and NNI ports of a PLSB Backbone Edge Bridge (BEB)would be separate physical interfaces. However, through VLANpartitioning of a physical interface, it is possible to envision thecoexistence of both types of interworking on a single interface. In theembodiment of FIGS. 1A-B, a set of two attachment points 18 areprovisioned to connect the VPLS domain to the PLSB domain. Eachattachment point comprises a respective VPLS PE gateway 8 interconnectedwith a PLSB gateway 6 to permit bi-directional frame transfer. Each VPLSPE gateway 8 serves as an end-point of LAN segments 10 defined withinthe VPLS domain 4, and so can send and receive traffic of the VPLSdomain 4. Similarly, the PLSB gateway 6 of each attachment point 18 canserve as an end-point (UNI interworking) or a transit point (NNIinterworking) for unicast and multicast paths defined within the PLSBdomain 2.

In both the UNI and NNI interworking cases, VPLS simply offers LANsegments 10, while their role in the network (i.e. UNI or NNI) isdetermined by how PLSB uses them. Traffic flows through the LAN segments10 are controlled by filtering imposed by PLSB in order to properlycontrol the connectivity and provide loop free operation. To facilitatethis operation, the LAN segment 10 is modelled in the PLSB domain as avirtual node 20.

UNI Interworking

Referring to FIG. 1A, in UNI interworking, the filtering is performed atthe PLSB service layer where the association of a frame with a specificService Instance Identifier (I-SID) is extended into the VPLS domain 4via the use of service tagging (typically I-SID to IEEE 802.1ad S-tagmapping) such that a single point of attachment for each serviceinstance can be enforced by filtering at the PLSB gateway 6 of eachattachment point 18. In order to enable PLSB to control traffic routingto or from the VPLS domain 4, each set of virtual LAN segments 10 isassociated with a respective set of I-SIDs, and represented asassociated with a virtual node 20 in the PLSB domain 2. The virtual node20 is modelled in IS-IS via gateway advertisements as being connected tothe respective PLSB gateway of each attachment point 18 via a virtuallink 22. With this arrangement, respective costs of the virtual links 22can be manipulated to force every path on the shortest path tree betweenthe set of LAN segments 10 associated with a virtual node 20 and everydestination address 14 in the PLSB domain 2 to traverse only one of theattachment PLSB gateways 6 at any given time. In the event that thepreferred path fails for whatever reason, normal IS-IS procedures willcause a different path to be computed to the virtual node 20, and theassociated forwarding state to be installed in the PLSB domain.

In embodiments in which a common VPLS LAN segment 10 is connected tomultiple points of attachment 18, frames injected into the VPLS LANsegment 10 at one point of attachment 18 will emerge at each of theother points of attachment 18, as may be seen in FIG. 3. However, asnoted above, by suitably manipulating the respective costs assigned toeach virtual link 20, the shortest path to or from the virtual node 20representing the VPLS LAN segment 10 will traverse only one PLSB gateway6, which is the preferred point of attachment 18 for all servicesassociated with that virtual node 20. Consequently, the PLSB gateways 6in the other attachment point(s) 18 will not contain forwarding statefor that VPLS LAN segment 10 either to or from the PLSB domain 2. Anyframes received from that VPLS LAN segment 10 at other than thepreferred point of attachment 18 for the service will simply bediscarded. Similarly any frame sent from VPLS into PLSB via thepreferred point of attachment 18 will not be reinserted back into theVPLS network 4 at any other points of attachment 18. PLSB itself willonly use the preferred point of attachment 18 for frames originatingoutside of VPLS which are sent to the VPLS domain 4.

In the embodiment of FIG. 1, the VPLS gateways 8 of both attachmentpoints 18 are connected to a single, multi-homed LAN segment 10. In thiscase, however, the conventional “broadcast on unknown destinationaddress” operation of the VPLS domain does not create any difficulties,because only the preferred PLSB gateway 6 will contain multicastforwarding state for any given I-SID in the PLSB domain 2 for the givenLAN segment 10.

Unlike H-VPLS or PBB front-ended implementations, where Spanning TreeProtocol (STP) would inefficiently disable the connection at all but onegateway for all traffic, PLSB only disables the connection at all butone gateway for traffic on a common VPLS LAN 10 associated with a set ofI-SIDs and represented as a virtual node 20 in the PLSB domain 2. Othersets of I-SIDs can each be represented in the PLSB domain 2 by arespective virtual node 20, and the virtual link costs to each virtualnode 20 can be independently manipulated as described above. Thus, byusing PLSB, efficient multi-homing of the interface between a VPLSdomain 2 and an external domain is enabled and the benefits of loadbalancing and resilient handover between multi-homed connections areenabled.

The nature of PLSB operation is that it selects a symmetric singleshortest path between any set of nodes and incorporates loop avoidancein the operation of the control plane to ensure the single path propertyis maintained through periods of network instability. Numerous otherlink state driven technologies upon which an Ethernet service can beoverlaid such as MPLS or IP can also benefit from this resilienceapproach providing accommodations are made for the more generalizedscenario of connectionless networking which permits simultaneousexistence of multiple paths to a given destination and does notinherently enforce single shortest path. In this scenario the techniqueof metric manipulation ensures that the preferred point of attachment isthe only node to which downstream traffic is directed by the link statenetwork. An additional coordination mechanism is also required betweenthe preferred point of attachment and the other nodes in the upstreamdirection to ensure that all but the preferred point of attachment blockupstream traffic as this will not be a property of normal networkoperation. In a preferred embodiment, a coordination channel is usedbetween the points of attachment in a “break before make” fashion toensure the points of attachment are synchronized prior to the blockingor unblocking of upstream ports and advertising changes into the linkstate control system. Such a channel may also be used for statesynchronization between the points of attachment when such additionalcoordination is required to have multiple nodes emulate attachment to avirtual node. An example of such information is VPLS label bindings.

NNI Interworking

In NNI interworking, all MAC addresses exposed to VPLS are B-MAC layeraddresses. MAC level filtering is directly controlled by PLSB at theedges of the VPLS domain. Each LAN segment 10 in the VPLS domain 4dedicated to PLSB operation appears in the PLSB topology via thetechnique of modelling the LAN segment 10 as a “virtual node” 20. Thispermits PLSB to compute paths across the network that include the VPLSLAN segments 10 as topological components and installing both forwardingand filtering state accordingly. The flooding of PLSB B-MAC addresses byVPLS will naturally be pruned by the filtering function at peer PLSBnodes connected to VPLS. This is facilitated by the fact that in a givenBackbone VLAN, PLSB operation dictates a single shortest path to or fromany given node in the network.

It is possible to achieve additional efficiencies in the NNIinterworking scenario. Referring to FIG. 1B, it can be seen that thebenefits of load balancing and resilient handover between multi-homedconnections can be provided with the implementation of only one VPLS LANsegment 10 dual-homed on the PLSB domain 2. However, recovery from anetwork failure (in either domain) requires that both domains 2,4compute new forwarding state. More particularly, PLSB must first computenew forwarding state to use an alternate PLSB gateway 6 to re-establishtraffic flow to or from the VPLS domain 2; and then VPLS must unlearnits previous forwarding state (e.g. via MAC withdraw/invalidationmessaging) and then learn new forwarding state. Since PLSB is a LinkState Protocol technology, recovery in the PLSB domain is relativelyfast. However, recovery in the VPLS domain tends to be much slower.

Referring to FIGS. 2 and 4, this problem may be overcome by provisioningtwo or more separate LAN segments 10 within the VPLS domain 4 dedicatedto PLSB operation. For example, a separate LAN segment 10 can beprovided in the VPLS domain 4 for each attachment point 18 to the PLSBdomain 2. In the example illustrated in FIG. 2, two LAN segments 10 a,10b are provided, each of which is single-homed onto one of the twoattachment points 18 to the PLSB domain 2. In the example illustrated inFIG. 4, two parallel LAN segments 10 a,10 b are provided, each of whichis dual-homed onto both of the two attachment points 18. Otherconfigurations are equally possible. A separate virtual node 20 a,20 bis provided for each LAN segment 10 a, 10 b. The PLSB domain 2 willchoose which LAN segment 10 to use for any given connection usingconventional shortest path calculations (e.g. IS-IS), and the respective“cost” assigned to each of the virtual links 22 a, 22 b. Because theVPLS LAN segments 10 “learn”, and the IS-IS protocol of PLSB overlaid onVPLS coordinates the surrounding PLSB nodes, the shifting ofconnectivity for a given MAC address from one VPLS LAN 10 to the otherdoes not involve the invalidation of any MAC information in the VPLSdomain 4. The MAC address is either known in the VPLS LAN segment 10currently used by PLSB or will promptly be learned. PLSB control planeexchange, computation and convergence ensures that the points ofattachment 18 agree on transit points for a given MAC address.

In some embodiments, the costs assigned to the virtual links 10 can bebased on an available capacity of the corresponding LAN segments 10, sothat PLSB path computations will “naturally” use the attachment point 18and LAN segment 10 on the unique shortest path for a given connection.In other cases, it may be advantageous to assign costs to the virtuallinks 22 is such a way as to force the PLSB path computation to select adesired attachment point 18 and/or LAN segment 10 for a connection.Examples using this latter alternative are described below.

Advantageously, PLSB will naturally utilize both LAN segments 10simultaneously, the degree depending on how IS-IS computation determinesthe paths through the PLSB network 2. A failure that impacts someportion of one VPLS LAN segment 10 will be reflected in PLSBcomputations and it will simply cause the affected portion of thetraffic matrix to be moved over to the other VPLS LAN segment 10. As theVPLS LAN segments 10 are parallel and distinct entities there is nothingfor VPLS to unlearn as a consequence of the transition; the new segmentwill simply learn the required connectivity, and what had already beenlearned on the old segment will merely be unused. As may be appreciated,this accomplishes the effect of protection switching, without requiringVPLS to perform any unlearning of connectivity via the failedconnection. This avoids the failure recovery delays and control planeoverhead inherent to normal VPLS operation.

As may be appreciated, improved load balancing can be provided by usingmultiple backbone VLAN IDs (B-VIDs) in the PLSB domain 2. For example,each LAN segment 10 may be associated with a respective B-VID in thePLSB domain 2. If each LAN segment 10 appears at all physicalinterfaces, then load balancing between each LAN segment 10 can beobtained based on PLSB equal-cost multi-path calculations, rather thanjust the unique shortest path to a given destination address.

The embodiment of FIG. 2 utilizes two virtual nodes; one for eachattachment point 18. This arrangement is beneficial in that it alsoenables load distribution, on a per I-SID basis, across all of theattachment points 18 between the VPLS and PLSB domains. However, it willbe appreciated that this is not essential. The same methods can equallybe employed using a single virtual node for handling traffic of all ofthe LAN service instances provisioned within the VPLS domain 4. Thus ingeneral, the number of virtual nodes 20 may be less than, equal to, orgreater than the number of attachment points 18, as desired.

It is also possible to envision an integrated node that performs controlplane exchange and direct interworking with both the VPLS and PLSBnetworks. The manner in which VPLS is modelled in the PLSB control planeis unaffected by this variation. Integrated nodes have sufficientinformation locally to prune the set of pseudo wires that multicastframes are replicated on. Integrated nodes and non-integrated nodes canbe combined in the same network. The preferred embodiment for resilienceis still two points of attachment between PLSB and VPLS domains and thismay be in the form of links between nodes that implement only one of thetwo technologies, or integrated nodes that perform both data plane andcontrol plane interworking.

The VPLS service model is that of an Ethernet LAN segment, so it ispossible to envision other technologies being substituted for VPLS as iswarranted by a combination of economics and operational behaviour. AProvider Bridged (802.1ad) network can be substituted directly for aVPLS network, although such a substitution is only envisioned asadvantageous where it is already deployed in the metro and isinterconnected with PLSB in a UNI interworking scenario.

The foregoing description describes methods of interfacing a PLSB domainwith a VPLS domain. Those of ordinary skill in the art will recognise,however, that these same techniques can be used to interface networkdomains configured under other protocols, so that the present inventionis not limited to PLSB and VPLS interfacing. In particular, PLSB is anexample of an Ethernet-based Link-State controlled protocol, whereasVPLS is a example of an Ethernet Bridging protocol. If used with theadditional coordination mechanisms described above in paragraph 0035,VPLS may alternatively be used to create a Link-State controlled networkdomain upon which Ethernet bridging is overlaid. Other knowntechnologies which may be used to create a Link-State controlled networkdomain upon which Ethernet bridging is overlaid include, for example,Transparent Interconnection of Lots of Links (TRILL). Further examplesof Ethernet-based Link-State control protocols include, for example,Link State Bridging, and also the Shortest Path Bridging (SPB) andShortest Path Backbone Bridging (SPBB) protocols being developed by theIEEE in the IEEE 802.1aq project. As such, it will be seen that thetechniques of the present invention can be used to interface anyEthernet-based Link-State controlled network domain or Link-Statecontrolled network domain upon which Ethernet bridging is overlaid withan Ethernet Bridging controlled network domain.

The embodiment(s) of the invention described above is(are) intended tobe exemplary only. The scope of the invention is therefore intended tobe limited solely by the scope of the appended claims.

1-23. (canceled)
 24. A method of providing an Ethernet service over anetwork comprising a Virtual Private LAN Service (VPLS) domain and aProvider Link State Bridging (PLSB) domain, the PLSB domain comprisingat least two PLSB networks, the method comprising: provisioning at leasttwo separate LAN segments dedicated to PLSB operation within the VPLSdomain; and connecting each LAN segment at most once to each of the PLSBnetworks.
 25. The method of claim 24, comprising representing eachvirtual LAN segment as a respective virtual node in the PLSB domain. 26.The method of claim 25, wherein each virtual node is modelled inIntermediate System to Intermediate System (IS-IS) protocoladvertisements as being connected to respective attachment points byrespective virtual links.
 27. The method of claim 24, wherein eachvirtual LAN segment is modelled in Intermediate System to IntermediateSystem (IS-IS) protocol advertisements as a virtual node connected torespective attachment points by respective virtual links.
 28. The methodof claim 24, comprising the PLSB domain selecting a respective LANsegment to be used for a connection based on a shortest pathcalculation.
 29. The method of claim 24, wherein: connecting each LANsegment at most once to each of the PLSB networks comprises connectingeach LAN segment to each of the PLSB networks by a respective link; andthe method comprises assigning a respective cost to each link connectingone of the LAN segments to one of the PLSB networks.
 30. The method ofclaim 29, comprising the PLSB domain selecting a respective LAN segmentto be used for a connection based on a shortest path calculation usingthe respective costs assigned to the links connecting the LAN segmentsto the PLSB networks.
 31. The method of claim 25, wherein connectingeach LAN segment at most once to each of the PLSB networks comprises:defining a respective virtual link to each virtual node in the PLSBdomain; and assigning a respective cost to each virtual link in the PLSBdomain.
 32. The method of claim 27, comprising the PLSB domain selectinga respective LAN segment to be used for a connection based on a shortestpath calculation using the respective costs assigned to the virtuallinks.
 33. The method of claim 24, comprising each VPLS LAN segmentlearning connectivity.
 34. The method of claim 33, wherein each VPLS LANsegment learning connectivity comprises each VPLS LAN segment learningforwarding information for PLSB B-MAC addresses from frames received bythe VPLS LAN segment from the PLSB domain.
 35. The method of claim 33,comprising changing connectivity for a particular MAC address in thePLSB domain from one LAN segment of the VPLS domain to another LANsegment of the VLPLS domain without invalidating any MAC forwardinginformation previously learned in the VPLS domain.
 36. The method ofclaim 33, comprising: changing connectivity for a particular MAC addressin the PLSB domain from a first LAN segment of the VPLS domain to asecond LAN segment of the VPLS domain; and the second LAN segment of theVPLS domain learning forwarding information for the particular MACaddress from frames received from the PLSB domain.
 37. The method ofclaim 36, wherein forwarding information for the particular MAC addressis not unlearned by the first LAN segment of the VPLS domain.
 38. Themethod of claim 24, comprising dual-homing at least one node of at leastone of the at least two PLSB networks onto the two separate LANsegments.
 39. The method of claim 24, comprising: configuring at leasttwo PLSB virtual LANs (VLANs) in the PLSB domain; and provisioning, foreach PLSB VLAN, at least two separate LAN segments dedicated to PLSBoperation within the VPLS domain.
 40. The method of claim 39, comprisingrepresenting each virtual LAN segment as a respective virtual node inthe PLSB domain.
 41. The method of claim 39, comprising: defining arespective virtual link to each virtual node in the PLSB domain; andassigning a respective cost to each virtual link in the PLSB domain. 42.The method of claim 41, comprising the PLSB domain selecting arespective LAN segment to be used for each virtual LAN based on ashortest path calculation using the respective costs assigned to thevirtual links.
 43. The method of claim 39, wherein: connecting each LANsegment at most once to each of the PLSB networks comprises connectingeach LAN segment to each of the PLSB networks by a respective link, eachlink connecting the at least two PLSB VLANs to at most one of theirrespective at least two separate LAN segments; and the method comprisesassigning a respective cost to each link connecting one of the LANsegments to one of the PLSB networks.
 44. The method of claim 43,comprising the PLSB domain selecting a respective LAN segment to be usedfor a connection based on a shortest path calculation using therespective costs assigned to the links connecting the LAN segments tothe PLSB networks.
 45. The method of claim 39, comprising the PLSBdomain balancing load on the LAN segments based on equal-cost multi-pathcalculations.
 46. The method of claim 24, comprising: configuring atleast two virtual LANs (VLANs) in the PLSB domain so that each VLANappears at all physical interfaces of the PLSB domain; and loadbalancing between the at least two LAN segments dedicated to PLSBoperation based on PLSB equal-cost multi-path calculations.